Glasses have sharp cut-on points at wavelengths in the vicinity of the border of the UV and visible ranges are known. They are used primarily as cut-off filter glasses. One technique for achieving this effect is the introduction into the glass of chromophoric colorants (i.e., chromophores) such as cadmium sulfide, cadmium selenide and cadmium telluride. See, e.g., W. Vogel, Chemistry of Glass, 163-173, American Ceramic Society, 1985.) The glasses become colored by "striking" of the base melts, e.g., by heat treatment at temperatures in the range of 550.degree.-850.degree. C. for times on the order of 30 min. up to 24 hrs. This heat treatment causes the formation of microcrystals which achieve the UV and/or short wavelength visible radiation cutoff and, hence, the color. A nucleation step before striking ensures uniform formation of crystallites and increases chances of reproducibility when the striking is performed in a continuous manner.
The cut-on wavelength can be varied by varying the heating conditions. See German Patent 1,596,981 and U.S. Pat. Nos. 3,773,530 and 4,106,946, which also exemplify the wide range of base glasses which are suitable.
Such struck glasses have been utilized for special uses such as traffic signal lenses for the German patent, optical filters, and shooter's (e.g., hunter's) eyeglass lenses, e.g., for S-7083 of Schott Glass Technologies Inc., Duryea, Pa. 18642, where the colors and transmittances producible in such glasses are appropriate. However, there are other important uses such as sunglasses, where these "struck" glasses have been inapplicable.